1. Field Of The Invention
The present invention relates to the treatment of air pollution. Specifically, though not exclusively, the present invention relates to an apparatus and method to control particulate air pollutants.
2. Problems In The Art
In facilities such as power plants or factories, the facilities may produce particulate air pollutants which may consist of solid material entrained in a gaseous medium, liquid particles, fine fume-type particles, various types of smoke, nuisance dust, or any type of suspended solid material which is not easily separated from the gaseous medium by gravitational force. Similarly, in industries such as food industries, pharmaceutical industries, or chemical industries may produce very fine powders which must by collected. Such dust may originate from a variety of sources including the combustion or thermal processing of fuels or waste materials, chemical processing, food processing, cement kilns, or powder handling, for example.
One prior art method of reducing the particulate air pollutants involves the use of a conventional pulse-jet baghouse. A typical pulse-jet baghouse includes a number of individual bags or filtration tubes which are four to six inches in diameter, eight to twenty feet long, and are mounted within and suspended from a tube sheet. The particulate dust is collected on the outside surfaces of the bags while the flue gas passes through the bag fabric to the inside, where it exits through the top of the bags into a clean air plenum and subsequently out the stack. Cages are installed inside the bags to prevent them from collapsing during the normal filtration process. To clean the bags, air nozzles are installed above each bag. By applying a quick burst of high pressure air directed inside the bags, the bags are cleaned. This burst of air causes a rapid expansion of 31 the bag and momentarily reverses the direction of gas flow through the bag, which helps to clean the dust off the bags. In a typical prior art baghouse, the bags are oriented in a rectangular array spaced a few inches apart. The bags are usually cleaned with the burst of air one row at a time in sequence, with approximately fifteen bags per row. As a result of the small bag spacing and forward filtration through the two rows of bags adjacent to the row being cleaned, much of the dust that is removed from one row of bags is simply recollected on the adjacent rows of bags. As a result, only the very large agglomerates of dust reach the hopper after supplying the burst of air through the bags. This phenomenon of redisbursion and collection of dust after bag cleaning is a major obstacle to operating prior art baghouses at higher filtration velocities, also known as air-to-cloth (A/C) ratio.
One prior art method of controlling particulate air pollutants is disclosed in U.S. Pat. No. 4,904,283 issued to Hovis et al. on Feb. 27, 1990. This prior art method integrates filtration and electrostatic precipitation in one step. A high voltage electrode is installed in the center of the filter bags, with the grounded electrodes woven into the bags. One major problem with this method is that there is no effective way of transferring the collected dust from the bags to the hopper without reentrainment and recollection.
Another prior art method of controlling particulate air pollutants is disclosed in U.S. Pat. No. 5,217,511 issued to Plaks et al. on Jun. 8, 1993. This method involves the placement of high voltage electrodes between pulse-jet-cleaned bags. Again, a major disadvantage of this method is that there is no effective way to transfer the dust from the bags to the hopper without reentrainment and recollection.
Another prior art method of controlling particulate air pollutants is disclosed in U.S. Pat. Nos. 5,024,681 and 5,158,580 issued to Chang on Jun. 18, 1991 and Oct. 27, 1992. This prior art method uses a high ratio fabric filter installed downstream from an electrostatic precipitator and includes the option of installing a separate precharger section between the precipitator and fabric filter. Again, there is no effective way for removing the dust from the bags without reentrainment and recollection.
Another prior art method of controlling particulate air pollutants is disclosed in U.S. Pat. No. 4,357,151 issued to Helfritch et al. on Nov. 2, 1982. This prior art method discloses a particulate collection method which uses electrostatic collection and filtration combined into the same housing in which high voltage electrodes are spaced between cylindrical perforated grounded surfaces as a filter outer shell and a pleated filter medium inside the cylindrical shell. This method has similar disadvantages.
Therefore, a need can be seen for an apparatus and method for effectively and efficiently controlling particulate air pollutants.